The present invention relates to a method and apparatus for manufacturing a semiconductor device and, more particularly, to a method and apparatus for manufacturing a semiconductor device using selective growth of a metal.
In recent years, the integration density of semiconductor devices is increased by reducing the sizes of constituent elements. However, as the device feature size shrinks, various problems are arising in manufacturing processes.
For interconnections, the interconnection width is reduced to make a small device. Generally, in the manufacture of a semiconductor device, a contact hole is formed in an insulating film to electrically connect upper and lower interconnections separated by the insulating film. This contact hole size must be reduced as the interconnection width decreases. For this reason, the aspect ratio (depth/diameter) of contact holes is becoming high.
Conventionally, sputtering is used to fill a contact hole with a conductive material. Sputtering is a film formation technique generally used to form a metal thin film. However, when sputtering is used to fill a contact hole having a high aspect ratio with a metal, step coverage may be degraded at the bottom or side portion of the contact hole, or a void may be formed in the contact hole to result in an increase in electrical resistance. Therefore, a demand is arising for a film formation technique that allows formation of reliable interconnections even when contact holes have a high aspect ratio.
As one of such film formation techniques, so-called selective CVD for depositing a desired metal only in a contact hole using chemical vapor deposition (to be referred to as CVD hereinafter) is known.
Selective CVD is a technique of depositing a desired metal interconnection material only on a metal surface using the fact that the decomposition properties of a source gas used for film formation on the metal surface are different from those on an insulating surface. The contact hole is filled with the metal material by growing the desired metal film only on the bottom portion of the contact hole, i.e., the exposed surface of the lower interconnection while controlling the substrate to a predetermined temperature. For example, in tungsten selective CVD for selectively filling a contact hole with tungsten (W), selective growth is performed using, as a source gas, a gas mixture of tungsten hexafluoride (WF.sub.6) and silane (SiH.sub.4) while controlling the substrate to a predetermined temperature.
In many cases, however, when the contact hole is filled with tungsten, tungsten particles are generated not only in the contact hole but also on other insulating film surface portions. This phenomenon is known well as "selectivity loss". If upper interconnections are formed in the subsequent process, the tungsten particles generated on the insulating film surface may cause short circuit between the upper interconnections to result in interconnection failures. This "selectivity loss" is a serious problem in bringing tungsten selective growth into practice.
The "selectivity loss" is a problem in selective CVD using another metal material as well which is supposed to realize selective growth based on a similar mechanism. For example, aluminum CVD using alkyl aluminum and copper CVD using acetylacetonate-olefin-copper are also regarded as techniques for realizing selective growth in principle. However, these selective CVD techniques of forming metal films have not been put into practice as a method of manufacturing a semiconductor device because of the "selectivity loss".
As described above, it is important in the metal CVD techniques to maintain high selectivity in growing a desired metal. For this purpose, an ideal insulating film surface where metal nucleation sites are hardly generated, i.e., an insulating film surface which does not receive/give electrons from/to source gas molecules adsorbed in the insulating film surface must be prepared. Conversely, in a region where the desired metal material is be selectively grown, the metal material must be easy to deposit, i.e., the source gas must be immediately decomposed to the desired metal.
In the actual semiconductor device manufacturing process, however, an ideal insulating film surface where "selectivity loss" does not occur and the ideal metal surface is hard to obtain. This will be described below while exemplifying tungsten selective growth.
Normally, reactive ion etching (to be referred to as RIE hereinafter) is used to form a contact hole. For this reason, RIE damage or residue of reaction product of a resist is generated on the bottom portion of the contact hole. In addition, when the semiconductor substrate (wafer) is conveyed in the atmosphere after formation of the contact hole, the bottom portion of the contact hole oxidizes. In this way, a so-called RIE contaminated layer is formed at the bottom portion of the contact hole. Such an RIE contaminated layer must be removed because it impedes tungsten growth in the contact hole.
A semiconductor substrate having a contact hole is normally processed using a plasma of an inert gas to remove the RIE contaminated layer. At this time, an insulating film of, e.g., SiO.sub.2 in which the contact hole is formed is also exposed to the plasma. Consequently, the surface of the insulating film is changed in properties and damaged.
If the surface of the SiO.sub.2 film is processed by an Ar plasma, the ratio of Si to O deviates from the stoichiometric ratio to generate excess Si atoms. The excess Si atoms form dangling bonds on the surface of the SiO.sub.2 film and activate the surface of the SiO.sub.2 film. WF.sub.6 or SiH.sub.4 molecules are readily trapped by the dangling bonds to generate nuclei for growing tungsten particles on the SiO.sub.2 film. Additionally, when the SiO.sub.2 film surface is modified with hydroxyl groups generated upon reaction between defects on the SiO.sub.2 film surface and residual water vapor in the vacuum chamber, nuclei for growing tungsten particles or tungsten nucleation on the SiO.sub.2 film are easily generated.
When the surface of the SiO.sub.2 film is activated, the selectivity of tungsten growth lowers. Thus, a technique of selectively growing tungsten while keeping the insulating film surface stable is desired.
As a method of removing the RIE contaminated layer, the semiconductor substrate is processed using a plasma of a halogen-based gas, as is known. In this method as well, the surface of the insulating film (SiO.sub.2 film) having the contact hole is exposed to the plasma, as in use of the inert gas plasma. Although the insulating film surface is damaged, the damaged layer is sequentially removed because the insulating film is simultaneously chemically etched by active halogen ions. Therefore, damage to the insulating film surface is less serious relative to the use of the inert gas plasma.
However, as semiconductor devices are increasing in integration density in recent years, the interconnection pitch has reduced to the submicron order, so higher selectivity is required. Even when the halogen-based gas plasma is used, sufficient selectivity cannot always be obtained.
In addition, since a halogen gas is used as an etching gas, the surfaces of lower interconnections at the bottom portion of the contact hole are converted into a halide. When the contact hole is filled with tungsten to form a tungsten plug, a halide layer or a layer containing a large quantity of halogen remains at the interface between the tungsten plug and the lower interconnection. This increases the contact resistance between the tungsten plug and the lower interconnection.
Furthermore, the halogen remaining between the plug and the lower interconnection corrodes the interconnection layer mainly containing aluminum upon annealing in the subsequent multilevel interconnection formation process or absorbing water from the ambient around the semiconductor substrate.
There is one more factor that lowers the selectivity in tungsten selective CVD. Tungsten is grown on a semiconductor substrate in a vacuum process chamber. The semiconductor substrate accommodated in the process chamber is heated to give rise to a chemical reaction for depositing tungsten only on the semiconductor substrate. However, it is difficult to exclusively heat the semiconductor substrate, and members in the process chamber, other than the semiconductor substrate, are also heated. Consequently, tungsten film formation reaction takes place on these heated members, too.
Tungsten grown on the members other than the semiconductor substrate results in dust. The dust becomes nuclei for growing tungsten. When the dust adheres to the surface of the insulating film formed on the semiconductor substrate, the selectivity lowers.
To prevent the selectivity from lowering due to dust, the interior of the process chamber is cleaned by a plasma using fluorine gas to remove tungsten adhering to the process chamber. In the process chamber, jigs contacting the semiconductor substrate are normally formed from silica. The silica jigs are damaged by this cleaning, so tungsten readily grows in the subsequent CVD process. Therefore, the surfaces of silica jigs in the process chamber are also required to be stable.
As described above, when a metal is to be grown by the conventional method, a metal film is also formed on the insulating film surface because of, e.g., dangling bonds on the insulating film surface. That is, "selectivity loss" readily occurs. When a metal is to be selectively grown using the conventional method, the metal film also easily forms on the surfaces of silica members in the process chamber.